To explore further the usefulness of opossum kidney (OK) cells in the study of renal dopaminergic physiology, we have undertaken the study of aromatic L-amino acid decarboxylase (AAAD), catechol-O-methyltransferase (COMT) and type A and B monoamine oxidase (MAO-A and MAO-B), the main enzymes involved in the synthesis and degradation of dopamine. The Vmax values for AAAD, using L-DOPA as the substrate, in rat renal tubular cells were found to be significantly (P < 0.01) higher (120-fold) than in OK cells. However, K(m) values in OK cells (1.1 mM [0.3, 1.9]) were similar to those observed in rat renal tubular cells (K(m) = 1.0 mM [0.8, 1.2]). The Vmax values for COMT (in nmol/mg protein/30 min) in OK cells (2.1 +/- 0.2) were similar to those in the rat renal tubular cells (1.6 +/- 0.1), whereas K(m) values in OK cells (2.3 microM [0.1, 4.5]) differ considerably (4.8-fold, P < 0.01) from those in rat renal tubular cells (11.2 microM [9.2, 13.1]). The Vmax values (in nmol/mg protein/20 min) for deamination of [3H]-5-hydroxytryptamine, the specific MAO-A substrate, was similar in rat renal tubular cells (12.4 +/- 1.0) and OK cells (12.9 +/- 1.1); K(m) values also did not differ between these two preparations. In contrast to rat renal tubular cells, deamination of [14C]-beta-phenylethylamine, the substrate for MAO-B, in OK cells was found to be non-saturable and to represent less than 10% of that observed in homogenates of rat tubular cells. In conclusion, OK cells in culture are endowed with the synthetic and metabolic machinery needed to form and degrade dopamine. The amounts of the enzymes AAAD, COMT and MAO-A found in this cell line are likely to be sufficient to reproduce, under in vitro conditions, the environment in which the renal dopaminergic system normally operates.